New Site Help

I want to use it for several purposes but I’m struggling as to how to move forward on each one and wonder if there is any docs I can read that will help. Here is what I want to achieve:
1. Link business in a group. Posts to the group get sent to each member. But limit those who can post to this group.
2. Link members of the public to sign up for a news group. This will allow businesses, charity and tourist information to post stuff to a group that will tell everyone about St Austell etc but again I want to only allow certain users to be able to post to this group.
3. I want to have 2 sign up forms, one that will link to users who will sign up at 1 and one that will sign up to no 2.

I should add that there are 2 levels of businesses – members and non-members. Non-members do not need a password sent to them just an email confirmation letter. And in respect of the 2nd group of people they just need an email confirmation letter.